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In Florida, screwworm moves inland from Keys

​Report finds infestation likely has human source

Posted March 1, 2017

The Agriculture Department’s Animal and Plant Health Inspection Service in January confirmed a report of New World screwworm infestation in a stray dog near Homestead, Florida, nearly a hundred miles northeast of where the parasite was discovered last October, more than 30 years since its eradication in the United States.

The dog was isolated and its wounds treated. This was the first confirmed case of screwworm infestation on Florida’s mainland. The parasite was found months earlier in more than 130 endangered Key deer from National Key Deer Refuge on Big Pine Key (see JAVMA, Jan. 15, 2017).

Also in January, APHIS released a report outlining the initial findings of its epidemiologic investigation of the Big Pine Key infestation. While no specific source was identified, the evidence strongly suggests humans introduced fly larvae to the island.

“This means that humans may have entered the country unknowingly infested with (New World screwworm) larvae, or they may have unknowingly brought animals infested with NWS larvae,” APHIS announced Jan. 6.

The report suggested that NWS was most likely introduced during the spring of 2016 in the lower Florida Keys, where Key deer provided a suitable host for the fly to become established.

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​Sterile screwworm flies (Courtesy of USDA APHIS 2016, PIO Pam Manns)

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Since the initial discovery, 13 keys have been documented to have screwworm infestations, mostly involving the Key deer population, with five confirmed infestations in domestic animals. Animal health and wildlife officials have been working aggressively to eradicate the pest. Extensive response efforts have included fly assessments to determine the extent of the infestation, release of sterile flies to prevent reproduction, and disease surveillance to look for additional cases in animals.

Officials have received substantially fewer reports of adult screwworm flies in the area and seen fewer cases of infected Key deer. By early January, fly assessments had been conducted on 40 keys.

Additionally, the USDA has released 112 million sterile flies from 34 ground release sites in the Florida Keys and the Homestead area, according to data provided Feb. 10 by the Florida Department of Agriculture and Consumer Services. During the 1950s, the department developed a method for eradicating the screwworm by releasing infertile flies into screwworm-infested areas. When they mate with wild females, no offspring result. With fewer fertile mates available in each succeeding generation, the fly breeds itself out of existence.

The USDA used this technique to eradicate screwworm from the U.S. and worked with other countries in Central America and the Caribbean to eradicate it there as well.

Human cases of infestation with New World screwworm are rare, but they have occurred. No human cases had been reported in Florida as of press time in February.

Additional information about the screwworm infestation is available here, including the report “Investigation into Introduction of New World Screwworm into Florida.”